Polyisocyanate, used as a raw material of polyurethane, is industrially produced, for example, by allowing polyamine to react with carbonyl chloride, for isocyanate reaction.
In this producing plant of polyisocyanate, after completion of the isocyanate reaction, high-molecular-weight polyisocyanate, which is a byproduct produced during the production, is separated from the crude polyisocyanate obtained.
In recent years, it has been proposed that the residues secondarily produced are continuously fed to a reactor vessel in its liquid state, while also high temperature and high pressure water is continuously fed to the reactor vessel, with temperature of the reactor vessel controlled to 190-300° C., whereby the residues are decomposed to polyamine so that the polyamine obtained can be reused (cf. Patent Document 1 cited below, for example).    [Patent Document 1] Japanese Unexamined Patent Publication No. 10-279539.